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Proper locations for foundation and basement slab insulation?

MJDesigns | Posted in Energy Efficiency and Durability on

If one is already planning R10 insulation outside the perimeter of the foundation wall such as Tremco’s Warm-n-Dri and their Horizon finishing system above grade, is it also a good idea to add rigid foam insulation on the inner side wall of the poured basement slab? Image is to have R10 subslab rigid insulation along with a shorter rigid insulation wall section (R10) along the perimeter of the basement slab. In other words, should the rigid be applied just outside the foundation or outside the foundation as well as the perimeter of the concrete floor. If the answer is yes to around and under the slab, how should the rigid pieces intersect … lay the side wall section on top of the sub slab section or along side of it? Is that detailed section available somewhere on the site? Is it preferable to spec XPS or a higher density EPS rigid product for that application?

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  1. jklingel | | #1

    Got a drawing of exactly what you have in mind? Covering the edge of the slab, if exposed, is critical. Are you insulating the stem wall all the way to the footer? How about around the footer? Under the footer?

  2. RMGheHhbJx | | #2

    Follow this link to see a good example of slab insulation for a basement or otherwise. This method completely de-couples the slab from the foundation wall and the ground.

    You will need to scroll down a ways to see the picture.

  3. MJDesigns | | #3

    Insulation of the foundation wall would be at the exterior all the way down to where it rests on the footer. I didn't consider insulating the side or bottom of the footer itself. As I'm reading, it sounds like the rigid insulation should also be on the inner side of the foundation wall alongside the concrete slab as well as underneath of it. XPS or 1.5lb. EPS OK? Location is central OH with just under 6000 HDD. No need to be concerned with termites and the rigid? Thanks again.

  4. jklingel | | #4

    First, I am not a pro. The "in or out" debate seems to be just that, and I don't think it matters. What does matter is to have continuous insulation as much as possible. Search for the Sunrise House here; Thorsten has everything wrapped up inside foam. Others do it inside, leaving the foundation wall outside, except at one corner where the foundation wall meets the wall insulation. (I'll put a pic of two options I've come up w/, shamelessly stealing some ideas from others. Note: Pics are not complete yet.) Intuitively, the foundation wall should be outside, as "Why heat the concrete all year?" However, I don't see mass in the heat loss equation, so once the concrete is up to temp, I don't see it as being a drain all year. Maybe I am missing something there. On your foam under the footer, if you chose to do that, be aware that the advertised psi rating is at either 5% or 10% compression, and once EPS gets into the 5-7% range, it permanently deforms. A rule of thumb is to go 3x what psi you figure you'll need; that is stated on the DOW web site, too, to eliminate "creep". (Ex: If your footer has 8 psi, use 25 psi foam.) My situation would call for using 25 psi EPS under the footer, but 40 costs so little extra that I think it is warranted. The highest psi EPS I can get is 60; 100 on the XPS. It seems to me that the trend is to go w/ EPS over XPS, too. Bugs love foam; protect it.

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